Design of MAC-defined aggregated ARQ schemes for IEEE 802.11n networks Kai-Ten Feng • Yu-Zhi Huang • Jia-Shi Lin Published online: 15 December 2010 Ó Springer Science+Business Media, LLC 2010 Abstract Based on the IEEE 802.11n standard, frame aggregation is considered one of the major factors to improve system performance of wireless local area net- works (WLANs) from the medium access control (MAC) perspective. In order to fulfill the requirements of high throughput performance, feasible design of automatic repeat request (ARQ) mechanisms becomes important for providing reliable data transmission. In this paper, two MAC-defined ARQ schemes are proposed to consider the effect of frame aggregation for the enhancement of net- work throughput. An aggregated selective repeat ARQ (ASR-ARQ) algorithm is proposed, which incorporates the conventional selective repeat ARQ scheme with the con- sideration of frame aggregation. On the other hand, the aggregated hybrid ARQ (AH-ARQ) protocol is proposed to further enhance throughput performance by adopting the Reed-Solomon block code as the forward error correction (FEC) scheme. Novel analytical models based on the signal flow graph are established in order to realize the retrans- mission behaviors of both schemes. Simulations are con- ducted to validate and compare the proposed ARQ mechanisms with existing schemes based on service time distribution. Numerical results show that the proposed AH-ARQ protocol outperforms the other retransmission schemes owing to its effective utilization of FEC mechanism. Keywords Wireless local area networks (WLAN) IEEE 802.11n standard Medium access control (MAC) Automatic repeat request (ARQ) Performance analysis 1 Introduction In recent years, the techniques for wireless local area net- works (WLANs) have been prevailing exploited for both indoor and mobile communications. The applications for WLANs include wireless home gateways, hotspots for commercial usages, and ad-hoc networking for inter-vehic- ular communications. Among different techniques, the IEEE 802.11 standard is considered the well-adopted suite due to its remarkable success in both design and deployment. Various amendments are contained in the IEEE 802.11 standard suite, mainly including IEEE 802.11a/b/g [1–3] and IEEE 802.11e [4] for quality-of-service (QoS) support. With increasing demands to support multimedia appli- cations, the new amendment IEEE 802.11n [5, 6] has been proposed for achieving high throughput performance. The IEEE 802.11 task group N (TGn) enhances the PHY layer data rate up to 600 Mbps by adopting advanced commu- nication techniques, such as multi-input multi-output (MIMO) technology [7]. It is noted that the MIMO tech- nique utilizes spatial diversity to improve both the range and spatial multiplexing for achieving higher data rate. However, it has been investigated in [8] that simply improves the PHY data rate will not be sufficient for enhancing the system throughput from the medium access control (MAC) perspective. Accordingly, the IEEE 802.11 TGn further exploits frame aggregation and block acknowledgement techniques [6, 9] to moderate the draw- backs that are originated from the MAC/PHY overheads. K.-T. Feng (&) Y.-Z. Huang J.-S. Lin Department of Electrical Engineering, National Chiao Tung University, Hsinchu, Taiwan e-mail: [email protected]Y.-Z. Huang e-mail: [email protected]J.-S. Lin e-mail: [email protected]123 Wireless Netw (2011) 17:685–699 DOI 10.1007/s11276-010-0307-6
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Design of MAC-defined aggregated ARQ schemes for IEEE802.11n networks
Kai-Ten Feng • Yu-Zhi Huang • Jia-Shi Lin
Published online: 15 December 2010
� Springer Science+Business Media, LLC 2010
Abstract Based on the IEEE 802.11n standard, frame
aggregation is considered one of the major factors to
improve system performance of wireless local area net-
works (WLANs) from the medium access control (MAC)
perspective. In order to fulfill the requirements of high
throughput performance, feasible design of automatic
repeat request (ARQ) mechanisms becomes important for
providing reliable data transmission. In this paper, two
MAC-defined ARQ schemes are proposed to consider the
effect of frame aggregation for the enhancement of net-
work throughput. An aggregated selective repeat ARQ
(ASR-ARQ) algorithm is proposed, which incorporates the
conventional selective repeat ARQ scheme with the con-
sideration of frame aggregation. On the other hand, the
aggregated hybrid ARQ (AH-ARQ) protocol is proposed to
further enhance throughput performance by adopting the
Reed-Solomon block code as the forward error correction
(FEC) scheme. Novel analytical models based on the signal
flow graph are established in order to realize the retrans-
mission behaviors of both schemes. Simulations are con-
ducted to validate and compare the proposed ARQ
mechanisms with existing schemes based on service time
distribution. Numerical results show that the proposed
AH-ARQ protocol outperforms the other retransmission
schemes owing to its effective utilization of FEC
mechanism.
Keywords Wireless local area networks (WLAN) � IEEE
802.11n standard � Medium access control (MAC) �Automatic repeat request (ARQ) � Performance analysis
1 Introduction
In recent years, the techniques for wireless local area net-
works (WLANs) have been prevailing exploited for both
indoor and mobile communications. The applications for
WLANs include wireless home gateways, hotspots for
commercial usages, and ad-hoc networking for inter-vehic-
ular communications. Among different techniques, the IEEE
802.11 standard is considered the well-adopted suite due to
its remarkable success in both design and deployment.
Various amendments are contained in the IEEE 802.11
standard suite, mainly including IEEE 802.11a/b/g [1–3] and
IEEE 802.11e [4] for quality-of-service (QoS) support.
With increasing demands to support multimedia appli-
cations, the new amendment IEEE 802.11n [5, 6] has been
proposed for achieving high throughput performance. The
IEEE 802.11 task group N (TGn) enhances the PHY layer
data rate up to 600 Mbps by adopting advanced commu-
nication techniques, such as multi-input multi-output
(MIMO) technology [7]. It is noted that the MIMO tech-
nique utilizes spatial diversity to improve both the range
and spatial multiplexing for achieving higher data rate.
However, it has been investigated in [8] that simply
improves the PHY data rate will not be sufficient for
enhancing the system throughput from the medium access
control (MAC) perspective. Accordingly, the IEEE 802.11
TGn further exploits frame aggregation and block
acknowledgement techniques [6, 9] to moderate the draw-
backs that are originated from the MAC/PHY overheads.
K.-T. Feng (&) � Y.-Z. Huang � J.-S. Lin
Department of Electrical Engineering, National Chiao Tung
k successfully transmitted packets with k � j. Therefore,
the PGF of service time distribution for a single jth packet
can be obtained as
Tj;nrðzÞ ¼ ð1� w00ÞzTDIFS H0ðzj�1ÞS0;j;nr
ðzÞ
þXRl�1
i¼1
ð1� w0iÞSi;j;nrðzÞ�
ð28Þ
Wireless Netw (2011) 17:685–699 693
123
Yi
l¼0
zTDIFS Hlðzj�1Þ !
Yi�1
m¼0
w0mFm;j;nrðzÞ
!" #
þYRl�1
i¼0
zTDIFS Hiðzj�1ÞYRl�1
m¼0
w0mFm;j;nrðzÞ ð29Þ
where w0m represents the failed transmission probability for
an MPDU containing nr RS blocks, i.e. w0m ¼Pnr
k¼1
p0m;j;kn0k, and Hlðzj�1Þ is obtained from (14) by substituting
z with zj�1
. As a result, the PGF TnrðzÞ of service time
distribution for the entire A-MPDU by aggregating j from ato b can be obtained as
TnrðzÞ ¼
Xb
j¼a
Pj � ½Tj;nrðzÞ� j ð30Þ
where Pj is acquired from (17). The validation and com-
parison for the service time distribution TnrðzÞ will be
performed in Sect. 4.
3.4 Iterative algorithm for proposed ASR-ARQ
and AH-ARQ schemes
In order to calculate the service time distributions as in (19)
and (30) for the proposed ASR-ARQ and AH-ARQ
schemes respectively, an iterative algorithm is required to
be exploited. The reason for the usage of iterative method
is that the service time distributions in (19) and (30) are
functions of steady state probability pj in (3), which is
derived from jj as the probability that there are j packets
arrived within the service time. However, it can be
observed from (2) that jj is a function of service time
distribution. Therefore, it is required to utilize the iterative
process in order to obtain the service time distributions.
The procedures of the iterative algorithm are listed as
follows:
1. The algorithm starts with the initial condition of
saturation case, i.e. ½p0p1. . . pK�1pK � ¼ ½00. . .01�.2. Calculate the service time distribution of either PGF
T(z) in (19) for the ASR-ARQ scheme or PGF TnrðzÞ in
(30) for the AH-ARQ algorithm.
3. It is assumed that 1 ls is taken as the sampling interval
for the iterative algorithm. From the PGF T(z) as in
(19) (or TnrðzÞ in (30)), its corresponding PMF T i can
consequently be obtained which denotes the probabil-
ity that the service time is i ls. Based on the
acquisition of T i, the probability jj as defined in (2)
can be approximated and obtained as jj ’P1
i¼0
T ie�ki ðkiÞ j
j! . By substituting the probability jj into (3),
the newly updated steady state probability vector
½p00p01. . .p0K�1p0K � can therefore be acquired.
4. Based on the updated p0j obtained from step 3, a new
service time distribution T 0ðzÞ can be recalculated via
(19) (or T 0nrðzÞ in (30)). The newly acquired PMF T 0i
will be compared with the previously computed T i
value that was obtained from step 2. In the case that the
difference between these two values are within a pre-
specified threshold, the iterative process will be
terminated. If not, the iterative procedure will be
continued and proceed from step 2.
4 Performance evaluation
In this section, the performance of proposed ASR-ARQ and
AH-ARQ schemes will be validated and compared via
simulations. The binary symmetric channel is assumed for
performance comparison under error-prone channels.
A system C/C?? network simulation model is constructed
by considering the access point (AP) based single-hop
communications. As shown in Table 1, the MAC-defined
parameters that are described in the IEEE 802.11n standard
is adopted in both the analytical models and the simulations.
The validation of proposed analytical models for both
the ASR-ARQ and AH-ARQ schemes are illustrated from
Figs. 4, 5, 6, and 7. For validation purpose, an AP and a
single user station is considered within the network. Fig. 4
shows the validation for the ASR-ARQ scheme via the
successful transmission probability pm,j,0 obtained from
(13) versus the number of retransmission stage m. Each
A-MPDU is aggregated with 10 MPDUs, i.e. j = 10; while
the size of each MPDU is configured as 1024 bytes. Four
different channel conditions are considered for model
validation, i.e. BER be ¼ 5� 10�5; 10�4; 2� 10�4; and
2.5 9 10-4. The results obtained from the analytical model
are denoted as ‘‘ana’’; while that from the simulations are
represented as ‘‘sim’’. It can be observed that the results
Table 1 Parameters for performance evaluation
Parameter Value
RTS packet size 20 Bytes
CTS packet size 14 Bytes
Block ACK (BA) packet size 32 Bytes
MAC header size 28 Bytes
Time duration of DIFS (TDIFS) 34 l s
Time duration of SIFS (TSIFS) 16 l s
Length of single time slot (r) 9 l s
Retry_Limit (Rl) 7
Minimum contention window size (W) 8
Maximum number of retransmission (M) 7
Data rate 24 Mbps
694 Wireless Netw (2011) 17:685–699
123
from both the analytical model and the simulations are
consistent with each other under the various BER values.
The successful transmission probability pm,j,0 is increased
as the BER value is decremented. Furthermore, with a
larger number of the backoff stage m, it is reasonable to
find that higher value of successful transmission probability
can be obtained. On the other hand, Fig. 5 illustrates the
successful transmission probability p0m;j;0 in (25) from
AH-ARQ algorithm versus the number of retransmission
stage m under BER be = 10-2. Four different aggregation
sizes are considered including j = 5, 10, 15 and 20. The
(255,223,16) RS code over GF(28) is constructed for the
proposed AH-ARQ scheme via the primitive polynomial
1þ x2 þ x3 þ x4 þ x8. Each 835-bytes MPDU within an
A-MPDU consists of three RS blocks which excludes the
header and delimiter blocks. It is observed that there exists
slight discrepancies between the analytical and simulation
results. The analytical results will have smaller successful
transmission probability comparing with that from the
simulation results due to the adoption of upper bound for
the decoding error probability B as defined in (20). Fur-
thermore, it is intuitive to find from Fig. 5 that larger
number of packet aggregation will result in excessive
number of retransmission stages.
Figures 6 and 7 respectively show the model validation
for service time distribution T(z) in (19) and TnrðzÞ in (30)
under different numbers of iterations. The iterative
0 5 10 15 2010−5
10−4
10−3
10−2
10−1
100
Number of Retransmission Stages
CD
F o
f Suc
cess
ful T
rans
mis
sion
Pro
babi
lity
BER = 5 * 10−5, sim
BER = 5 * 10−5, ana
BER = 10−4, sim
BER = 10−4, ana
BER = 2 * 10−4, sim
BER = 2 * 10−4, ana
BER = 2.5 * 10−4, sim
BER = 2.5 * 10−4, ana
Fig. 4 Performance validation for ASR-ARQ scheme: successful
transmission probability pm,j,0 versus number of retransmission
stage (m)
0 5 10 15 2010−6
10−5
10−4
10−3
10−2
10−1
100
Number of Retransmission Stages
CD
Fof
Suc
cess
ful T
rans
mis
sion
Pro
babi
lity
j = 5, ana j = 5, sim j = 10, ana j = 10, sim j = 15, ana j = 15, sim j = 20, ana j = 20, sim
Fig. 5 Performance validation for AH-ARQ scheme: successful
transmission probability p0m,j,0 versus number of retransmission stage
(m)
100 1010
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (ms)
CD
F o
f Ser
vice
Tim
e D
istr
ibut
ion
IT−1, anaIT−2, anaIT−3, anaIT−4, anaIT−5, anasim
Fig. 6 Performance validation for ASR-ARQ scheme: service time
distribution T(z) under different numbers of iterations
100 101
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (ms)
CD
F o
f Ser
vice
Tim
e D
istr
ibut
ion
IT−1, anaIT−2, anaIT−3, anaIT−4, anaIT−5, anasim
Fig. 7 Performance validation for AH-ARQ scheme: service time
distribution TnrðzÞ under different numbers of iterations
Wireless Netw (2011) 17:685–699 695
123
algorithm as presented in Subsect. 3.4 is utilized for
obtaining the proposed analytical models to be compared
with the simulation results. The number of aggregated
packets varies within the range of [a, b] = [5, 10]
according the minimum batch rule, the arrival rate k is set
to 100 fps, and the channel condition is chosen as BER
be = 2 9 10-4. It can be observed from both figures that
the service time distributions obtained from the simulations
and analytical models are close with each other after sev-
eral iterations. The iterative process is converged after the
third iteration for the ASR-ARQ scheme, i.e. the analytical
results for IT =3,4, and 5 are overlapped as in Fig. 6. On
the other hand, the recursive process for the AH-ARQ
algorithm converges after the fourth iteration as shown in
Fig. 7. The slight differences between the simulation and
the analytical results can be explained from both the
approximation of several parameters and the assumption of
Markov ergodic process within the adopted queuing
models.
Figures 8 and 9 illustrate the performance comparison
for the mean service time and the normalized throughput
under different BER values respectively. It is noticed that
the mean service time is computed as the averaged value
from the service time distribution T(z) in (19) and TnrðzÞ in
(30) for the proposed ASR-ARQ and AH-ARQ scheme,
respectively. The normalized throughput is defined by
dividing the throughput with the data rate, i.e. 24 Mbps in
this case, where the throughput is designed as the total
number of successfully received information bytes within
the duration of mean service time. An aggregated stop-and-
wait ARQ (ASW-ARQ) scheme is also implemented in the
simulations for comparison purpose. It is noticed that the
ASW-ARQ approach is equivalent to adopting an ARQ
scheme within the aggregated MAC service data unit
(A-MSDU). As is defined in the IEEE 802.11n standard, an
A-MSDU indicates that an MPDU is composed by multiple
MSDUs, where a common CRC check and a MAC header
are shared by all the MSDUs within an A-MSDU. For the
purpose of fair comparison, the information payload is
designed as 657 bytes for all the three algorithms; while 10
data units are aggregated in these schemes, i.e. 10 MPDUs
for both the ASR-ARQ and AH-ARQ algorithms and 10
MSDUs for the ASW-ARQ scheme. Two different num-
bers of stations, i.e. NoS = 6 and 18, are simulated to
contend for the channel in order to communicate with the
AP. It can be observed from both figures that the proposed
AH-ARQ scheme outperforms the ASR-ARQ and ASW-
ARQ algorithms under different BER values and numbers
of contending stations. Persistent lowered mean service
time can be achieved by adopting the AH-ARQ scheme up
to BER be = 5 9 10-3; while that from the other two
algorithms increase significantly for BER value greater
than be = 5 9 10-6. On the other hand, the normalized
throughput for the AH-ARQ method can be retained up to
be = 10-3; while that from the other two schemes decrease
drastically after the BER value is larger than be = 10-5. It
is noted that the inferior performance resulting from the
ASR-ARQ and ASW-ARQ algorithms is primarily caused
by excessive failed retransmissions from packet collisions.
Moreover, as shown in Fig. 8, since comparably longer
contention period and more collision probabilities can
occur, additional access time will be required in all three
schemes while there exists more stations contending in the
network. The throughput performance will therefore be
decreased with the case of NoS = 18 comparing to that
with NoS = 6 as shown in Fig. 9.
1e−2 5e−3 1e−3 5e−4 1e−4 5e−5 1e−5 5e−6 1e−610−4
10−3
10−2
10−1
100
BER
Nor
mal
ized
Thr
ough
put
ASW−ARQ, NoS = 6ASR−ARQ, NoS = 6AH−ARQ, NoS = 6ASW−ARQ, NoS = 18ASR−ARQ, NoS = 18AH−ARQ, NoS = 18
Fig. 9 Performance comparison: normalized throughput versus BER
values (10 aggregated data units)
10−6
10−5
10−4
10−3
10−2
101
102
103
BER
Mea
n S
ervi
ce T
ime
(ms)
ASR−ARQ, NoS = 6ASW−ARQ, NoS = 6AH−ARQ, NoS = 6ASR−ARQ, NoS = 18ASW−ARQ, NoS = 18AH−ARQ, NoS = 18
Fig. 8 Performance comparison: mean service time (ms) versus BER
values (10 aggregated data units)
696 Wireless Netw (2011) 17:685–699
123
Figures 10 and 11 shows the performance comparison
for both the mean service time and normalized throughput
with 50 aggregated data units. Noted that a total of 50
MPDUs are aggregated for both the ASR-ARQ and
AH-ARQ schemes; while 50 MSDUs is utilized for the
ASW-ARQ algorithm. Similar performance results can be
obtained from Figs. 10 and 11 comparing with Figs. 8 and
9 respectively. Compared to the other methods, the pro-
posed AH-ARQ scheme can achieve lowered mean service
time and higher normalized throughput. It is worthwhile to
notice that there is no definite superiority on throughput
performance with the increased number of aggregated data
units. By observing Figs. 9 and 11, both the ASR-ARQ and
ASW-ARQ algorithms result in worse normalized
throughput under the case with 50 aggregated data units
compared to that with 10 aggregated data units, e.g. under
the cases with be = 10-5 and be = 5 9 10-6. The reason
can be explained from the tradeoff between the access delay
and the effective transmitted information payloads. With
larger number of aggregated data units, longer transmission
time will be required which results in elongated mean ser-
vice time. Therefore, by adopting the ASR-ARQ and ASW-
ARQ schemes, it is required to investigate the feasibility of
increased number of aggregated data units in order to pro-
mote the throughput performance under various BER con-
ditions. On the other hand, elongated transmission delay
associated with larger number of aggregated MPDUs does
not have severe impact on the throughput performance for
the AH-ARQ scheme. Based on its adoption of FEC tech-
nique, retransmission probability can be reduced which
effectively enhances the normalized throughput of AH-
ARQ algorithm. The merits of the proposed AH-ARQ
scheme can therefore be observed.
5 Conclusion
In this paper, two ARQ mechanisms are proposed with the
consideration of frame aggregation under the IEEE
802.11n networks. The aggregated selective repeat ARQ
(ASR-ARQ) protocol is proposed by incorporating the
conventional selective repeat ARQ scheme; while the
aggregated hybrid ARQ (AH-ARQ) algorithm further
enhances throughput performance by adopting the Reed-
Solomon block code for error correction. Analytical mod-
els based on signal flow graph are constructed to evaluate
the performance of proposed ASR-ARQ and AH-ARQ
algorithms. Simulations are also conducted to validate and
compare the effectiveness of proposed ARQ mechanisms
via both the mean service time and throughput perfor-
mance. Numerical results show that the proposed AH-ARQ
protocol can outperform the other retransmission schemes
under different network scenarios.
Acknowledgment This work was in part funded by the Aiming for
the Top University and Elite Research Center Development Plan,
NSC 99-2628-E-009-005, NSC 98-2221-E-009-065, the MediaTek
research center at National Chiao Tung University, the Universal
Scientific Industrial (USI) Co., and the Telecommunication Labora-
tories at Chunghwa Telecom Co. Ltd, Taiwan.
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